The great Notes application is now officially part of trom.tf/ - you should read more about it here trom.tf/notes/ to understand how to quickly set it all up. It works online or offline, and on any operating system. It syncs between them all and is auto-backed-up in the cloud and offline. Not to mention it is fully encrypted. Plus, you can use whatever text editor you prefer. If you write books, this is great for you. If you write simple notes, it is perfect. If you want to have to-do lists or tasks, again it meets your needs. Code? Again, you can do that too. Markdown support...Vim editor...you name it :). Plus, lots of themes and other extensions.
This Notes app normally costs 10 dollars a month (standardnotes.com/features) but we provide all of these features trade-free. So take advantage of that!